It was discovered at least as early as 1950 that if one uses a chamber bounded on one end by an acoustic wave generating device and on the other end by a rigid wall with a small orifice, that when acoustic waves are emitted at high enough frequency and amplitude from the generator, a jet of air will be produced radiating from the orifice outward from the chamber. See Ingard and Labate, Acoustic Circulation Effects and the Nonlinear Impedance of Orifices, The Journal of the Acoustical Society of America, March, 1950. However, the concern of scientists at that time was only with the relationship between the impedance of the orifice and the "circulation" created at the orifice. There was no suggestion to combine the apparatus with another fluid stream in order to modify that stream's direction of flow. Furthermore, there was no suggestion that such an apparatus could be used in such a way as to create a fluid flow in a bounded volume. Such uses and combinations were not only not suggested at that time, but also have not been suggested by any of the ensuing work in the art.
So, even though a crude synthetic jet was known to exist, applications to common problems associated with other fluid flows or with lack of fluid flow in bounded volumes were not even imagined, much less suggested. Evidence of this is the persistence of certain problems in various fields which have yet to be solved effectively.
Until now, the direction of a fluid jet has chiefly been controlled by mechanical apparatus which protrude into a jet flow and deflect it in a desired direction. For example, aircraft engines often use mechanical protrusions disposed in jet exhaust in order to vector the fluid flow out of the exhaust nozzle. These mechanical protrusions used to vector flow usually require complex and powerful actuators to move them. Such machinery often exceeds space constraints and often has a prohibitively high weight. Furthermore, in cases like that of jet exhaust, the mechanism protruding into the flow must withstand very high heat. In addition, large power inputs are generally required in order to intrude into the flow and change its direction. For all these reasons, it would be more desirable to vector the flow with little or no direct intrusion into the flow. As a result, several less intrusive means have been developed.
In an effort to avoid the problems associated with physical intrusion into a flow, a common method of vectoring a fluid jet is to use another fluid flow as a fluidic control jet. Use of a fluidic jet to vector another flow, while known for years, has been used with little success. In particular, the only way known to vector one jet of fluid with another jet of fluid was to align the control jet so as to impinge on the primary jet directly. This involved injection of mass into the flow and was not very effective at vectoring the primary flow because a direct momentum transfer was relied upon for the direction change. To be at all effective, therefore, the control jet needed to be operated at considerable power. Further, such control was difficult and expensive to install because of the complex plumbing necessary to supply the control jet with fluid to operate.
It has been known for a number of years that substantial modification of shear flows can result from the introduction of small perturbations at the boundaries of the shear flow. This modification occurs because the shear flow is typically hydrodynamically unstable to these perturbations. Such instability is what leads to the perturbations' rapid amplification and resultant relatively large effect on the flow. However, these principles have not been used to vector a fluid flow with any great degree of effectiveness.
Not only is it important for safety or thrust vectoring to control the direction of a fluid flow, but fluid flow about an aerodynamic surface is of great concern. In fact, modification of the aerodynamic characteristics of lifting surfaces has long been a goal of many scientists and manufacturers. Flaps and slats have proven successful in altering the apparent shape and camber of an aerodynamic surface, causing the free stream fluid flow to conform to the new shape. However, because of the complex control system required, flaps are expensive to manufacture and install. Flap systems are not only complex, but usually need considerable space on the interior of the aerodynamic surface for installation. Furthermore, considerable power input is required to operate a flap system because of the mechanical intrusion into a flow. Finally, flaps, being complex mechanical apparatus, are often quite heavy. In some applications, the weight penalty to be paid may destroy the usefulness of the flap altogether.
Other efforts of designers to modify the flow about an aerodynamic surface have centered on injection of energy into the boundary layer of the flow in order to augment lift, reduce drag, delay turbulence onset, or delay flow separation. For example, the method disclosed by U.S. Pat. No. 4,802,642 to Mangiarotty involves the retardation of a flow's transition to turbulence. However, this prior art does not and cannot change the effective aerodynamic shape of the airfoil. That is, the apparatus cannot change the direction of flow of the free stream fluid about the surface. Instead, the apparatus propagates acoustic excitation above the Tollmien-Schlichting frequency in an attempt to disrupt Tollmien-Schlichting waves as they begin to form and thereby delay the onset of turbulence. Although this method changes the drag characteristic of a lifting surface, the mean velocity field and thus apparent aerodynamic shape of the surface remain unchanged.
Such devices as slots and fluid jets have also been extensively employed to inject energy into the boundary layer in order to prevent flow separation. Recently, efforts have turned to the use of piezoelectric or other actuators to energize the boundary layer along an aerodynamic surface. See, for example, U.S. Pat. No. 4,363,991 to Edleman. These techniques, which employ acoustic excitation, change the surface aerodynamic performance by suppressing the naturally occurring boundary layer separation. This method requires the flow state to be vulnerable to specific disturbance frequencies. Although proving to be effective at delaying flow separation, none of these devices alter the apparent aerodynamic shape or mean velocity field of a given aerodynamic surface. Even though the changes in lift and drag that are caused by separation can be somewhat restored, there is no effect before separation occurs and the locus of the stagnation points remain largely unchanged. Therefore, before the present invention, there was no way to alter the effective shape of an aerodynamic surface without the complexity, high expense, and weight penalty of mechanical flaps or slats.
In a somewhat different field of study, the ability to effectively control the direction of a fluid stream may have great impact on the mixing of a fluid jet with the ambient fluid into which it is ejected. For example, mixing a hot exhaust plume with cold ambient air to shorten the downstream distance at which it is safe to work is a potential application.
In free turbulent shear flows mixing takes place within a shear layer between two nominally uniform streams of fluids moving at different speeds. In our example, the hot exhaust and the ambient air move at different speeds and mixing occurs at the boundary of these two flows. This boundary forms the turbulent flow region known as a shear layer. Hydrodynamic instabilities in this shear layer induce a hierarchy of vortical structures. Mixing begins with the entrainment of irrotational fluid from each stream by the large-scale vortical structures. Layers of the entrained fluid are continuously stretched and folded at decreasing scales by vortical structures that evolve through the action of shear and localized instabilities induced by larger vortical structures. This process continues until the smallest vortical scales are attained and fluid viscosity balances the inertial forces. This smallest vortical scale is referred to as the Kolmogorov scale. Thus, a long-held notion in turbulence is that the smallest and largest turbulent motions are indirectly coupled through a cascade of energy from the largest to successively smaller scales until the Kolmogorov scale is reached and viscous diffusion can occur. Turbulent dissipation is the process by which (near the Kolmogorov scale) turbulent kinetic energy is converted into heat as small fluid particles are deformed.
Scalar mixing (of heat or species, for example) is similar, but not identical to momentum mixing. Analogous to the Kolmogorov scale, the Batchelor scale is the smallest spatial scale at which an isoscalar particle can exist before scalar gradients are smoothed by the action of molecular diffusion. If scalar diffusion occurs on a faster scale than momentum diffusion, the Kolmogorov energy cascade breaks packets of scalars down into scales small enough that molecular scalar diffusion can occur (at the Batchelor scale). In this case, the Batchelor scale is larger than the Kolmogorov scale and turbulent motions persist at scales where scalar gradients have been smoothed out by diffusion. If, on the other hand, scalar diffusion occurs on a slower scale than momentum diffusion, turbulent motions stop (at the Kolmogorov scale) before the scalar gradients are smoothed out. Final mixing only occurs after laminar straining further reduces the size of the scalar layers.
Because the hierarchy of vortical structures within the shear layer results from flow instabilities, introduction of controlled disturbances at the flow boundary can be used to effectively manipulate a mixing process. To most effectively control mixing processes in turbulent shear flows, it is highly desirable to control both the large scale entrainment of fluid and the small scale mixing of that fluid. However, control of the small scale mixing until now, has been indirect, relying on manipulation of global two-dimensional and three-dimensional instability modes of the base flow to trigger the breakdown to small scale motion. Because this approach depends on the classical cascading mechanism to transfer control influence to the molecular mixing scales, mixing at the smallest scales is only weakly coupled to the control input. More importantly, mixing control of this nature relies on a priori knowledge of the flow instabilities and associated eigenfrequencies of the particular flow. Specifically, it also requires that the flow be unstable to a range of disturbances, a condition which is not always satisfied. In summary, more efficient control of mixing could be achieved by direct, rather than hierarchical, control of both the large scale entrainment and the small scale mixing. Such a control method has, before now, not been available and is enabled by synthetic jet actuators that are the subject of the present disclosure.
Creating a flow in a bounded volume is very desirable in some situations. Particularly, effective mixing of fluids inside a bounded volume could be achieved without the addition of new species, need for a fluid source or drain, and without a mechanical stirring device, which may require a large power input and place additional geometric constraints on the designer. Flow in a bounded volume is also beneficial to heat transfer processes, such as cooling in a sealed environment.
Some common applications of mixing in a bounded volume are mixing in chemical lasers and mixing for chemical or pharmaceutical products. In addition to these fields, the development of methods for enhancement of mixing through manipulation of the flow in which it occurs will have a direct impact on the performance of various other technologically important systems.
As mentioned above, mixing processes in virtually any technological application take place within plane or axisymmetric turbulent shear flows such as shear layers, wakes, or jets. This includes mixing in a bounded volume by use of a fluid flow or a stirring device. To effectively control mixing processes in turbulent shear flows, it is highly desirable to control both the large scale entrainment of fluid and the small scale mixing of that fluid. However, control of the small scale mixing, until now, has been indirect, relying on the classical cascading mechanism to transfer control influence to the molecular mixing scales. The various problems associated with this reliance were outlined above. Even though more efficient control of mixing can be achieved by direct control of both the large scale entrainment and the small scale mixing, until now, such a control method was not available, especially for bounded volumes.
Effective cooling of heated bodies in closed volumes has also been a long standing problem for many designers. Generally, cooling by natural convection is the only method available since forced convection would require some net mass injection into the system, and subsequent collection of this mass. The only means of assistance would be some mechanical fan wholly internal to the volume. However, often this requires large moving parts in order to have any success in cooling the heated body. These large moving parts naturally require high power inputs. But, simply allowing natural convective cooling to carry heat from the body producing it into the fluid of the volume and then depending on the housing walls to absorb the heat and emit it outside the volume is a poor means of cooling.
Thus, a heretofore unaddressed need exists in the art for apparatuses and techniques to better control the direction of a fluid flow and to produce a fluid flow in a bounded volume. The invention disclosed herein meets these heretofore unmet needs.